Recently, attention is focused on hybrid vehicles and electric vehicles as vehicles taking into account environmental issues. A hybrid vehicle includes, in addition to a conventional engine, a DC (direct current) power source, an inverter, and a motor driven by the inverter as the power source. In addition to achieving the power source by driving the engine, the DC voltage from the DC power source is converted into AC (alternating current) voltage by the inverter, and the converted AC voltage is used to rotate the motor to achieve power.
An electric vehicle includes a DC power source, an inverter, and a motor driven by the inverter as the power source.
A motor drive device incorporated in such a hybrid vehicle or electric vehicle employs the damping control technique to suppress vehicle vibration caused by deviation in torque control by accurately matching the output torque of the motor with the torque command (for example, refer to Japanese Patent Laying-Open No. 2005-198402). According to this publication, the torque command applied to the motor drive device is an addition of damping torque that is generated based on the waviness component of the motor revolutions or the like with the output torque primarily required of the motor. By controlling the motor drive with the added result as the eventual torque command, the pulsation component of the torque is canceled out. As a result, vehicle vibration can be suppressed.
In order to drive the motor at high efficiency, some types of hybrid vehicles are configured to allow adjustment of the applied voltage for motor drive (hereinafter, also referred to as “motor drive voltage”) according to the motor operating state (the number of revolutions, torque, and the like) by incorporating a level conversion function of DC voltage applied to the motor drive device that controls the motor drive. Particularly, by incorporating a booster function to increase the motor drive voltage higher than the input DC voltage, the battery qualified as a DC voltage source can be reduced in size. Further, power loss in association with the increased voltage can be reduced to allow higher efficiency of the motor.
Japanese Patent Laying-Open No. 10-066383, for example, discloses a configuration in which the motor for vehicle running is controlled. The DC voltages from a battery is boosted by a voltage-up converter to generate a motor drive voltage, which is converted into AC voltage by the inverter to be used for motor drive control. In accordance with this configuration, the target value of the motor drive voltage, i.e. the voltage command of the voltage-up converter, is determined based on the motor revolution and the torque command.
Consider the case where a torque command is used to determine the voltage command of the voltage-up converter during execution of damping control set forth above. The torque command in damping control corresponds to the primarily required torque added with the damping torque, and has a varying waveform reflecting the waviness component of the number of revolutions. Therefore, the voltage command determined based on such a torque command will vary, likewise the torque command. Such variation in voltage command will cause the voltage-up converter to frequently repeat a voltage-up operation and a voltage-down operation. As a result, the power loss occurring at the voltage-up converter increases to degrade the system efficiency of the motor drive device.
In accordance with the configuration set forth above in Japanese Patent Laying-Open No. 10-066383, a smoothing capacitor must be provided at the output side of the voltage-up converter to stabilize the motor drive voltage. Therefore, variation in the voltage command will cause a change in the holding voltage of the smoothing capacitor, such that the stored power will also vary. If a voltage-up operation and voltage-down operation are frequently repeated according to variation in the voltage command, the voltage-up converter is subject to variation of the stored power of the smoothing capacitor in addition to the consumed/generated power of AC motor M1. As a result, a relatively large current will flow through the voltage-up converter. If this current becomes excessive, the switching element constituting the voltage-up converter may be damaged. Further, increase of the current flowing through the voltage-up converter will increase power loss, and may become the cause of preventing high efficiency of the motor.